Change references to version 1.5.0 to version 2.0.0 (keyring branch)

[MatthewBennington]

We originally intended keyrings to be released in version 1.5.0. The release was delayed to accommodate spec clarifications, and now will include breaking changes.

Therefore, keyrings will be released in version 2.0.0.

• All changes originally included in 1.5.0 should now be bundled under 2.0.0.
• All mentions of 1.5.0 should be removed.

[MatthewBennington]

Also update the __version__ value in identifiers.py (currently 1.4.1 in keyrings branch).

[mattsb42-aws]

We have historically only updated __version__ value immediately prior to a release. This is sub-optimal but was the easiest thing to do.

I would suggest that we update it to 2.0.0b1. This both has the benefit of clearly identifying everywhere that uses the version that this is not a GA release and gives us the option to publish a beta release of 2.0.0 if we need/want to.

[farleyb-amazon]

No longer relevant, resolving.